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A Case
for the
Deep Ocean
Lisa A. Levin,
Nadine Le Bris, Erik Cordes,
Yassir Eddebbar, Rachel M. Jeffreys,
Kirk N. Sato, Chih-Lin Wei
and the climate change work group of
DOSI Deep-Ocean Stewardship Initiative
Encompassing 95% of the planet's habitable volume, the deep ocean plays a major role in climate
regulation, and thus new pressures exerted on it and its components should be a major concern.
The deep sea provides many ecosystem services, such as storing heat and anthropogenic carbon
emitted into the atmosphere. These services are key to sequestrating CO2 and CH4 on longer timescales, as well as in supporting nutrient cycling on which the entire foodweb (and so commercial
activities such as fisheries) relies. Heat absorption and redistribution impacts human exploitedspecies ranges. Already absorbing many pollutants and waste, the deep ocean could also become
a place for new activities such as mining. Setting up key mitigation and adaptation measures to
climate change will require new knowledge, and implementation of a complete legal framework
and management tools.
Covering over half of the planet, and comprising 95%
of its habitable volume, the deep ocean (>200 m)
merits dedicated attention in the context of climate
change for several major reasons:
• The deep ocean has a predominant role in the
•
•
sequestration of heat and carbon, with tight links to
the upper ocean and atmosphere through vertical
mixing, species migrations and particulate sinking,
and a diverse range of ecosystems, making it
critical to any analysis of ocean roles in climate
mitigation and adaptation.
The deep ocean provides a broad range of
ecosystem services that are just beginning to be
inventoried; greenhouse gas regulation, support
to biodiversity (including genetic diversity), food
supply and energy production.
The deep ocean is increasingly impacted by human
activities including contaminant inputs, overfishing,
and disturbances from seafloor extractive activities.
There is currently little understanding of how these
direct impacts will interact with climate stressors.
WE NEED TO INCREASE OUR
KNOWLEDGE TO BETTER PROTECT
THE DEEP OCEAN
Key adaptations to climate change will require new
knowledge, including the broadening of deep-water
observation programs, to enable the design of marine
protected areas encompassing vulnerable regions in
deep waters, and to inform environmental management
of industrial activities and development of new policies
addressing deep national and international waters.
There is an unprecedented need to integrate the deep
ocean into ocean science and policy. Knowledge of
deep hydrology, hydrography, pelagic and seafloor
ecology is critical to climate predictions and societal
impact assessments (e.g., Mora et al. 2013) because of the
strength of connectivity between the oceans, atmosphere
and the terrestrial realm. New international regulations (e.g.
for mining) and treaties (e.g. for biodiversity), environmental
management, and spatial planning also must incorporate
climate and the role of deep processes.
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ocean-climate.org
KEY ELEMENTS TO UNDERSTAND
THE FUTURE OF THE DEEP OCEAN
We draw attention to the following themes, which
make the case for the significance of the deep ocean.
Ecosystem services of the deep ocean
Life in the deep ocean provides or regulates many
valuable services that sustain the planet (Armstrong
et al. 2012; Thurber et al. 2014); key among these
are CO2 and CH4 sequestration, nutrient cycling,
substrate, food and nursery grounds provisioning
for fisheries by a variety of habitats. The deep
ocean is the largest reservoir of carbon on Earth and
constitutes the ultimate sink for most anthropogenic
carbon. The biogenic deep-sea carbon component
is poorly quantified, but chemosynthetic ecosystems
with high carbon fixation rates and vertical transport
by pelagic species may significantly contribute to
‘blue carbon’ sequestration (Marlow et al. 2014,
Trueman et al. 2014, James et al. 2016).
Thermal energy budgets
The ocean absorbs 90% of the extra heat trapped
by anthropogenic greenhouse gas emissions, with
30 % of this being stored at depths >700 m (IPCC
5th assessment report) and is thus a more accurate
indicator of planetary warming than surface global
mean temperature (Victor and Kennel, 2015). In this
stable and mainly cold environment (except in the
Mediterranean Sea and at bathyal depths in tropical
regions), thermal limits shape species distributions.
The consequences of warming on deep ocean waters
will profoundly influence ecosystems and their
biodiversity. Examples of rapid changes in deepsea benthic ecosystems have been documented
in downwelling, upwelling and polar regions (e.g.
Danovaro et al. 2004, Smith et al. 2012, Soltwedel
et al. 2016), although discriminating natural cycles
from climatic impacts in the deep sea will require
unprecedented time series data (Smith et al. 2013).
Biogeochemical changes
The deep ocean supports major biogeochemical
recycling functions; these are expected to undergo
major changes. Declines in O2, pH and aragonite
saturation have been observed and are predicted
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to strongly impact intermediate water depths
under future emission scenarios (Bopp et al. 2013).
Deep-water oxygenation is tightly coupled to the
overturning circulation and O2 trends inform changes
in global or basin-scale ocean circulation. As a
regulator of the biogeochemical cycling of N, Fe, P,
and S, O2 is key to potential synergistic responses.
N2O production is expected to increase as oxygen
declines (Codispoti, 2010), potentially linking O2
decline and climate through a positive feedback,
though large uncertainties remain (Martinez-Rey et
al. 2015).
Cumulative impacts of changes
There are many climate change-related stressors
affecting deep-sea ecosystem functions (Levin
and Le Bris 2015). Deep-sea ecosystems may
be particularly vulnerable to change due to their
environmental stability or to tight links with surface
productivity or hydrodynamic regime. Deep-sea
diversity patterns are shaped by export production
(Woolley et al. 2016) and CMIP5 models predict
overall decreases in integrated primary productivity
with climate change. Large reductions in the tropics
and the North Atlantic (Bopp et al. 2013), suggest
possible negative impacts for deep-sea diversity.
We need to assess how and where these cumulative
changes, including warming, ocean acidification,
aragonite undersaturation, shifts in nutrient fluxes
and deoxygenation, will challenge ecosystem
stability and species capacity to adapt (Lunden et
al. 2014, Gori et al. 2016). This involves gathering
sufficient knowledge about cumulative impacts of
multiple stressors to build accurate scenarios of
vulnerability.
A need for deep observations
The sparse nature and typically small spatial
resolution of deep-ocean observations, combined
with overly large spatial resolution of models,
results in knowledge gaps and uncertainties. This
includes natural variability, the coupling of climate
to biogeochemical cycles, and the responses of
biodiversity hotspots (e.g., seamounts and canyons).
In addition, multicellular life in the deep pelagic
realm is still largely unexplored, though this realm
represents over 95% of the living space on the
ocean-climate.org
planet. Seafloor observatories and long-term time
series have started providing insights into how deepsea ecosystems respond to climate perturbations
(Soltweddel et al. 2016; Smith et al. 2013). Longterm integrated ecological studies, covering a
range of deep-sea systems and the most vulnerable
hotspots, are needed to identify threats to critical
ecosystem services and the potential feedbacks to
the climate system and humans.
deep-ocean ecosystems are facing an onslaught
from pollutants, fishing, mining, energy extraction,
and debris (Mengerink et al. 2014), with deep
seabed mining now on the near-term horizon. New
efforts to develop requirements for environmental
impact assessments, environmental indicators,
spatial planning and create marine protected areas
in deep water will need to incorporate the interplay
with climate change.
Synergies of direct human-induced stressors
Beyond the complexity of multiple climate stressors,
Figure from Levin and Le Bris 2015. Winners and losers from exposure to interacting climate stressors. (A) King crabs invading Palmer Deep in Antarctica enabled by warming (9). (B) Cold seep fauna may expand as warming promotes methane
release from the seafloor (12), such as occurs at sites recently discovered along the Atlantic coast. (C) Hypoxia-tolerant
Humboldt squid (Dosidicus gigas) have extended their distribution in concert with expanding oxygen minima along the
East Pacific margin. (D) Cold-water coral reefs vulnerable to warming and acidification in Mediterranean canyons.
[Photo credits: (A) Image courtesy of K. Heirman and C. Smith, NSF LARISSA and Ghent University HOLANT projects. (B) Image courtesy of Deepwater
Canyons 2013–Pathways to the Abyss, National Oceanic and Atmospheric Admin- istration (NOAA)–Office of Exploration and Research, Bureau of Ocean
Energy Management, and U.S. Geological Survey. (C) Image courtesy of N. Le Bris, Laboratoire d'Ecogéochimie des Environnements Benthiques (LECOB),
Fondation Total– UPMC. (D) Image courtesy of R. Starr, NOAA–Cordell Bank National Marine Sanctuary].
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